This invention relates to an image pickup device coupled to, for example, a prescribed optical image transmission device to produce predetermined video signals, and more particularly to a television camera apparatus jointly used with a fiberscope to carry out a particular function such as an iris servo control.
As used herein, the term "iris servo function" is defined to mean a function of generating a video signal always having a prescribed voltage level, regardless of the brightness of a foreground subject whose image is to be picked up. This iris servo function fundamentally corresponds to the electric eye (EE) mechanism of the ordinary camera. The iris servo device of the conventional television camera generally comprises the undermentioned servo system. Namely, the conventional iris servo device detects the average voltage level or peak level of video signals obtained from a television camera. The iris mechanism of the television camera is servo-controlled in accordance with the result of said detection, thereby controlling the brightness of a foreground subject entering the target plane of the image pickup device of the television camera to a fixed level.
Hitherto, an automatic gain control (AGC) circuit has been used as means for electrically fixing the voltage level of the aforesaid video signal. Another known process of controlling the voltage of the video signal to a fixed level comprises the joint use of the AGC circuit and the aforementioned mechanical servo device. In this case, however, the mechanical servo device and AGC circuit are actuated independently of each other. In other words, while the mechanical servo loop is operated, the electric servo loop (AGC circuit) does not work. Conversely where the electric servo loop (AGC circuit) is operated, the mechanical servo loop becomes inoperative. The reason for the application of this procedure is that the independent actuation of the two servo loops enables a stable, accurate servo function to be more easily realized. It is known that a foreground subject whose image is projected on the image pickup device is preferred to have as great a brightness as a video signal-processing circuit is not saturated. The reason for this is that the greater the brightness of light entering the image pickup device, the higher the voltage level of a signal supplied to the video signal-processing circuit with the corresponding improvement in the signal to noise (S/N) ratio.
However, with the conventional image pickup device provided with an iris servo system consisting of the mechanical servo loop and electric servo loop, it is necessary for the operator to unfailingly determine a timing in which the operation of said two servo loops should be changed over in order to obtain a large S/N ratio over a broad range of the brightness of a foreground subject. However, this requirement demands the television camera operator to carry out a time-consuming and initiating changeover operation.